707 research outputs found
Frequency resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state
Using the RXTE/PCA data we study the fast variability of the reflected
emission in the soft spectral state of Cyg X-1 by means of Fourier frequency
resolved spectroscopy. We find that the rms amplitude of variations of the
reflected emission has the same frequency dependence as the primary radiation
down to time scales of <30-50 msec. This might indicate that the reflected flux
reproduces, with nearly flat response, variations of the primary emission. Such
behavior differs notably from the hard spectral state, in which variations of
the reflected flux are significantly suppressed in comparison with the primary
emission, on time scales shorter than ~0.5-1 sec. If related to the finite
light crossing time of the reflector, these results suggest that the
characteristic size of the reflector -- presumably an optically thick accretion
disk, in the hard spectral state is larger by a factor of >5-10 than in the
soft spectral state. Modeling the transfer function of the disk, we estimate
the inner radius of the accretion disk R_in~100R_g in the hard and R_in<10R_g
in the soft state for a 10M_sun black hole.Comment: submitted to MNRA
Soft state of Cygnus X-1: stable disk and unstable corona
Two component X-ray spectra (soft multicolor black body plus harder power
law) are frequently observed from accreting black holes. These components are
presumably associated with the different parts of the accretion flow (optically
thick and optically thin respectively) in the vicinity of the compact source.
Most of the aperiodic variability of the X-ray flux on the short time scales is
associated with the harder component. We suggest that drastically different
amplitudes of variability of these two components are simply related to the
very different viscous time scales in the geometrically thin and geometrically
thick parts of the accretion flow. In the geometrically thin disks variations
of viscosity or mass accretion rate occurring at large radius from the black
hole on the local dynamical or thermal time scales do not cause any significant
variations of the mass accretion rate at smaller radii due to a very long
diffusion time. Any variations on the time scales shorter than the diffusion
time scale are effectively dampened. On the contrary such variations can easily
survive in the geometrically thick flows and as a result the mass accretion
rate in the innermost region of the flow will reflect modulations of the mass
accretion rate added to the flow at any distance from the black hole. Therefore
if primary instabilities operate on the short time scales then the stability of
the soft component (originating from the geometrically thin and optically thick
flow) and variability of the hard component (coming from the geometrically
thick and optically thin flow) are naturally explained.Comment: 8 pages; accepted for publication in MNRAS; replaced with accepted
versio
Confinement and diffusion time-scales of CR hadrons in AGN-inflated bubbles
While rich clusters are powerful sources of X-rays, gamma-ray emission from
these large cosmic structures has not been detected yet. X-ray radiative energy
losses in the central regions of relaxed galaxy clusters are so strong that one
needs to consider special sources of energy, likely AGN feedback, to suppress
catastrophic cooling of the gas. We consider a model of AGN feedback that
postulates that the AGN supplies the energy to the gas by inflating bubbles of
relativistic plasma, whose energy content is dominated by cosmic-ray (CR)
hadrons. If most of these hadrons can quickly escape the bubbles, then
collisions of CRs with thermal protons in the intracluster medium (ICM) should
lead to strong gamma-ray emission, unless fast diffusion of CRs removes them
from the cluster. Therefore, the lack of detections with modern gamma-ray
telescopes sets limits on the confinement time of CR hadrons in bubbles and CR
diffusive propagation in the ICM.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations
We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation
from the Earth's atmosphere. We find that the shape of the spectrum emergent
from the atmosphere in the energy range 25-300 keV is mainly determined by
Compton scatterings and photoabsorption, and is almost insensitive to the
incident cosmic-ray spectrum. We provide a fitting formula for the hard X-ray
surface brightness of the atmosphere as would be measured by a satellite-born
instrument, as a function of energy, solar modulation level, geomagnetic cutoff
rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of
the atmospheric hard X-ray flux during the occultation of the cosmic X-ray
background by the Earth agrees with our prediction within 10%. This suggests
that Earth observations could be used for in-orbit calibration of future hard
X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by
cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly
different from that emitted by the Earth's atmosphere.Comment: 12 pages, 16 figures, MNRAS accepte
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